The present application is claiming priority of German Patent Application Serial No. 101 27 449. 1, which was filed on Jun. 7, 2001.
1. Field of the Invention
The invention concerns an illumination system for wavelengths of xe2x89xa6100 nm, wherein the illumination system has an object plane and a field plane.
2. Description of the Prior Art
In order to allow for further reduction in the structural widths of electronic components, especially in the submicron range, it is necessary to shorten the wavelength of the light used for microlithography. It is conceivable to use light with wavelengths of less than 100 nm, for example, lithography with soft x-rays, so-called EUV lithography.
EUV lithography is one of the most promising future lithography techniques. At present, wavelengths in the range of 11-14 nm, especially 13.5 nm, are being considered as wavelengths for EUV lithography, with a numerical aperture of 0.2-0.3. The image quality in EUV lithography is determined, on the one hand, by the projection objective, and, on the other hand, by the illumination system. The illumination system should provide the most uniform possible illumination of the field plane, in which the pattern-bearing mask, the so-called reticle, is situated. The projection objective images the field plane in an image plane, the so-called wafer plane, in which a light-sensitive object is arranged. Projection illumination systems for EUV lithography are designed with reflective optical elements. The form of the field of an EUV projection illumination system is typically that of an annular field with a high aspect ratio of 2 mm (width)xc3x9722-26 mm (arc length). The projection systems usually operate in scanning mode. Regarding EUV projection illumination systems, reference is made to the following publications:
W. Ulrich, S. Beiersdxc3x6rfer, H. J. Mann, xe2x80x9cTrends in Optical Design of Projection Lenses for UV and EUV Lithographyxe2x80x9d in Soft X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Publisher), Proceedings of SPIE, Vol. 4146 (2000), pages 13-24, and
M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizinga, xe2x80x9cIllumination Optics Design for EUV Lithographyxe2x80x9d in Soft X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Publisher), Proceedings of SPIE, Vol. 4146 (2000), pages 25-34,
whose disclosure content is incorporated to the full extent in the present application.
In illumination systems for wavelengths of xe2x89xa6100 nm, a problem arises in that a light source of such illumination system emits radiation of wavelengths that can lead to an unwanted exposure of the light-sensitive object in the wafer plane of the projection exposure system and, moreover, optical components of the exposure system, such as multilayer mirrors, are heated by such radiation.
In order to filter out radiation of unwanted wavelengths, transmission filters, made for example from zirconium, are used in illumination systems for wavelengths of xe2x89xa6100 nm. Such filters have the disadvantage of large losses of light. Furthermore, they can be very easily disrupted by thermal stress.
The object of the invention is to provide an illumination system for wavelengths of xe2x89xa6100 nm, especially in the EUV range, in which it is possible to avoid the aforesaid drawbacks and disadvantages. Furthermore, the components of such an illumination system should be simple in construction and manufacture.
According to the invention, this object is solved by an illumination system that has at least one grating element, which has a plurality of individual gratings with a grating period assigned to the individual gratings, and with at least one diaphragm in a diaphragm plane, which is arranged after the grating element in a beam path from an object plane to a field plane.
Grating elements, such as reflection gratings, in particular echelette gratings with an overall efficiency near 60%, are already known for some time from the design of monochromators for synchrotron radiation sources, and there is good experience available, especially even with very high fluxes.
The behavior of diffraction gratings is described by the grating equation       n    ⁢          xe2x80x83        ⁢          λ      p        =            cos      ⁢              xe2x80x83            ⁢              α        t              -          cos      ⁢              xe2x80x83            ⁢              α        i            
with a grating period p, a diffraction order n, an angle of incidence xcex1i relative to a surface tangent, an angle of diffraction xcex1t relative to the surface tangent, and a wavelength xcex.
When considering convergent or divergent radiation, it is necessary to consider an optical effect of the grating.
Regarding the use of diffraction gratings in monochromators, reference is made to the following publications, whose disclosure contents are fully incorporated in the present application:
H. Petersen, C. Jung, C. Hellwig, W. B. Peatman, W. Gudat: xe2x80x9cReview of plane grating focusing for soft x-ray monochromatorsxe2x80x9d, Rev. Sci. Instrum. 66(1), January 1995
M. V. R. K. Murty: xe2x80x9cUse of convergent and divergent illumination with plane gratingsxe2x80x9d, Journal of the Optical Society of America, Vol. 52, No. 7, July 1962, pp. 768-773
T. Oshio, E. Ishiguro, R. Iwanaga: xe2x80x9cA theory of new astigmatism and coma-free spectrometer, Nuclear Instruments and Methods 208 (1993) 297-301
The inventors have discovered for the first time that a grating element can be used in a beam path from an object plane to an image plane for spectral filtering in an illumination system for wavelengths of xe2x89xa6100 nm when individual diffraction orders and wavelengths are distinctly separate from each other.
The separation of the different diffraction orders is simple in a focused beam. In a focused beam at a focal point there is a focus or image of a light source with limited diameter. However, one needs to select a certain aperture for the focused beam, so as not to get excessively long structural lengths of the illumination system. On the other hand if the one selects a high aperture, the grating design is more difficult, or one gets larger aberrations.
To achieve separation of the individual diffraction orders the grating element is of complicated construction, for example, with a continuously varying grating constant or an arrangement on a curved surface. A substantial effort is required to produce such gratings.
The inventors have recognized that a separation of the individual diffraction orders and sufficient imaging quality can also be achieved if one uses a plurality of individual gratings, instead of a grating element with continuously changing grating constant, for example.
Preferably the individual gratings are arranged either one on top of another or one behind the other in a direction of an impinging beam. The individual gratings in a first embodiment of the invention can be gratings with different grating periods.
In an alternative embodiment, individual gratings are arranged tilted relative to each other.
When the individual gratings are arranged one after the other, cooling devices can be provided at a side facing away from the impinging beams. In this manner, it is possible to prevent undesirable heating of the grating.
The individual gratings are preferably designed as Blaze gratings, which are optimized for maximum efficiency in one diffraction order. Blaze gratings are known, for example, from the Lexicon of Optics, published by Heinz Hagerborn, pages 48-49. They are distinguished by an approximately triangular groove form of the grating.
As already mentioned, the different diffraction orders and wavelengths can be distinctly separated from each other with the grating element according to the invention.
The at least one diaphragm according to the invention serves to prevent stray light with wavelengths far above 100 nm from getting into the illumination system via the 0th diffraction order. The at least one diaphragm basically blocks the light of the 0th diffraction order.
It is especially preferable for rays after the diaphragm to have wavelengths in the region of 7 to 25 nm, by a combination of grating and a diaphragm.
Advantageously, the illumination system includes a collector unit to generate a convergent bundle of light and the convergent light bundle, sometimes referred to as a beam bundle, impinges on the grating element.
Especially preferred, a focus of the light bundle of the nth diffraction order, where |n|xe2x89xa71, of the grating element will come to lie at a site of the diaphragm or in a vicinity of the diaphragm,
In order to avoid an excessive thermal load on the diaphragm in a diaphragm plane or on optical elements that come after it, a portion of radiation of unwanted wavelengths can be filtered out by additional diaphragms in the illumination system.
Besides the illumination system, the invention also provides a projection exposure system with such an illumination system, as well as a method for the production of microelectronic components.